1. Field of the Invention
An aspect of the present invention relates to an image forming apparatus and a toner adhesion amount correction method.
2. Description of the Related Art
As a color image forming apparatus of an electronic picture system, a tandem system is known where an image forming part for each of the colors yellow (Y), magenta (M), cyan (C) and black (K) is arranged along a belt-shaped intermediate transfer body.
The image forming part for each color includes a photoreceptor and a charger, an exposure unit, a developing unit and a photoreceptor cleaner arranged around the photoreceptor. The surface of the photoreceptor is uniformly charged by the charger and the charged part is exposed by laser beams emitted from the exposure unit. This forms an electrostatic latent image on the photoreceptor and a toner image corresponding to the electrostatic latent image is formed by the developing unit.
When the intermediate transfer body is conveyed to the first image forming part (yellow), the toner image on the photoreceptor is transferred. The intermediate transfer body is sequentially fed to the other image forming parts, where toner images of other colors are transferred one over another. The photoreceptor where transfer is over has unnecessary toner remaining on its surface. The unnecessary toner is removed by the photoreceptor cleaner for formation of a next image.
In this way, toner images of respective colors formed one over another are transferred to print paper as a recording material and the print paper with the toner images fixed thereon is ejected.
In such an image forming apparatus, degradation or a secular variation of an image carrier including a photoreceptor, an intermediate transfer body and a recording material results in a smaller amount of toner adhesion to the image carrier thus causing degradation in the printing quality. For example, JP-A-2003-186278 and JP-A-2006-84796 disclose methods for correcting toner adhesion amount.
FIG. 5 illustrates an exemplary toner adhesion amount correction method according to the related art in cure paper printing.
The toner adhesion amount correction method corrects toner adhesion amount based on the detection result of an adhesion amount sensor 12 for printing out a correction toner image 15 inside the maximum form area 5B in a direction orthogonal to the carrying direction of an intermediate transfer body 5 (direction of width of the intermediate transfer body 5) at the start of printing and optically measuring the toner adhesion amount of the correction toner image 15. FIG. 5 also show a photoreceptor 2.
In FIG. 5, a sign 5A represents the outer region the maximum form area 5B. The maximum form area 5B refers to the maximum print width where toner image transfer to a recording material is available in the width direction of the intermediate transfer body 5. The outer region 5A of the maximum form area 5B refers to the region of an intermediate transfer body not involved in toner image transfer to the recording material in the width direction of the intermediate transfer body 5.
The toner adhesion amount correction method in the cut paper printing assumes that the image forming area on the intermediate transfer body 5 includes a space between pages. The method forms a correction toner image 15 to correct the toner adhesion amount in the center of the space between pages and performs correction control in accordance with a detection signal from the adhesion amount sensor 12.
In case a web that is continuous in the shape of a belt is used as a recording material and continuous printing is made on the web, no space between pages exists in the image forming area on the intermediate transfer body 5. Thus, printing on a web must be suspended in order to print a correction toner image 15 inside the maximum form area 5B on the intermediate transfer body 5 and optically measure the correction toner image 15 to correct the toner adhesion amount. In other words, when a correction toner image 15 is formed inside the maximum form area 5B of the intermediate transfer body 5 while continuous web printing is on the way, it is not possible to remove the correction toner image 15 before transfer to the web, thus staining the web. As a result, the printing efficiency drops.
A method is also known for printing a correction toner image 15 in the outer region 5A of the maximum form area on the intermediate transfer body 5 to solve the problems.
In this case, it is possible to perform correction control of toner adhesion amount during continuous web printing. That is, based on a detection signal from the adhesion amount sensor, setting of the charging voltage is corrected with timing that the position corresponding to a space between pages reaches just below the charger, and setting of the development bias voltage is corrected with timing that the position reaches just below the developing unit, and setting of the exposure amount is corrected with timing that the space reaches just below the exposure unit to make toner adhesion amount correction. With this method, it is possible to make control of the toner adhesion amount without lowering the printing efficiency.
However, in the related art, a difference between the toner adhesion amount outside the maximum form area on the intermediate transfer body and that inside the intermediate transfer body in order to print a correction toner image, if any, could affect the printing quality. This is mainly due to the fact that, when a secondary transfer is made from an intermediate transfer body to a web during continuous web printing, the resistance value of the intermediate transfer body is subjected to a secular variation since the intermediate transfer body has a surface that comes into contact with the web and another that does not. When the resistance value of the intermediate transfer body changes, the transfer efficiency of the first transfer differs between outside the maximum form area and inside the maximum form area, thus resulting in a difference in the toner adhesion amount.
As described in JP-A-2006-84796, there is proposed an image forming apparatus that regularly performs image quality control based on a correction toner image formed in the regions at both ends of the web in its width direction and that performs image quality control with the position where the correction toner image is formed switched in the center of the web in its width direction when the measurement value of the correction toner image is out of a permitted range.
In the case of JP-A-2006-84796, printouts from the image forming apparatus include a page including a correction toner image formed in each of the regions at both ends of the web in its width direction and a page including a correction toner image formed in the center of the web in its width direction.
It is thus necessary to store the history of switching the position where a correction toner image is formed and switch web cutting processing based on the history information. This lowers the printing efficiency.